The cooling air in a gas turbine engine is subject to windage as it flows through the internal air system. The work in this thesis focuses on the windage generated as the cooling air passes over the rotor surface, particularly for case where bolts are encountered. Reducing windage heating of the cooling air is of great importance to turbomachinery engineers, particularly in the aerospace industry, since the use of compressor air for cooling greatly reduces the thrust potential of an engine. The ability to accurately predict windage can help reduce the quantity of cooling air required, resulting in increased efficiency. A purpose built rig was used to measure both windage and rotor surface temperature as air passes through an enclosed rotor-stator cavity. A wide range of flow conditions were tested with some being close to those found in a modern gas turbine engine. A variety of both stator and rotor mounted bolts were investigated, of varying size and shape, as well as cavities in the disc surface. In addition, PIV measurements of the core tangential velocity were obtained. Test results show that windage is increased substantially with rotor bolts present, compared with a plain disc, and that it increases with increasing bolt size. For hexagonal rotor bolts a new correlation was produced between the moment coefficient and bolt diameter to pitch ratio for a range of flow conditions, characterised by the rotational and throughflow Reynolds numbers. Stator bolts were shown to generate a large increase in disc surface temperature compared with the plain disc at engine representative conditions. PIV measurements of the core tangential velocity showed an increase of up to 80% above the plain disc with rotor bolts present and no superimposed flow. When throughflow was introduced, the increase was around 300%. These measurements also demonstrate a local increase in tangential velocity in the region close to the bolt.